Strengths and Limitations of NOAA ESRL Measurements of Atmospheric CH4 for Source Attribution

Direct and indirect components to anthropogenic radiative forcing by atmospheric CH4 are estimated to be ~0.7 W m-2, or about half the contribution of CO2. Through its chemistry, methane also affects the abundance of tropospheric ozone, a strong oxidant and greenhouse gas. It is estimated that the increase in atmospheric CH4 abundance over the past 200 years is responsible for half the increase in background tropospheric ozone levels; moderate ozone levels affect human respiratory function, while higher levels lower agricultural crop yields and damage natural ecosystems. Policies aimed at mitigating the potential environmental effects of atmospheric CH4 require a detailed understanding of the global CH4 budget by emission sector and how emission rates are changing with time. During the past 2 decades, the globally averaged CH4 growth rate decreased from ~14 ppb yr-1 in 1984 to near zero recently. Since 1999, the global methane budget appears to be at steady state, with emissions and sinks each at ~550 Tg CH4 yr-1. This large-scale constraint on the global CH4 budget is one of many strengths of the GMD CH4 measurements. Even at regional scales, the data can usefully constrain emissions. For example, analysis of changes in the N/S gradient of atmospheric CH4 indicate that widespread concerns of substantially increased emissions from melting permafrost have not yet been realized, and increases in these emissions on order of 10 Tg CH4 yr-1 can be observed with the existing network. On the other hand, our sampling network is less sensitive to CH4 emissions in the tropics. CH4 emissions from vegetation, particularly in the tropics, is a potentially large, newly discovered source. Other recent work suggests more CH4 is emitted from the Amazon Basin relative to the Pantanal region in South America than is found in commonly used emissions distributions. In these cases, the ESRL surface measurements are limited by relatively few sites, all far from source regions, and strong vertical transport that quickly dilutes surface signals. The ESRL surface measurements will continue to provide strong constraints on the global CH4 budget, but further advances in understanding regional scale emissions, especially in the tropics, will require satellite measurements such as those from AIRS and SCIAMACHY. Satellite measurements will be especially useful when they are validated by GMD vertical profiles, especially those in the tropics.